1. Field of the Invention
The present embodiments relate to aligning radio frequency pulse timing characteristics.
2. Related Art
There are many applications where precision alignment of radio frequency pulse timing characteristics are required. As merely one example, when a multiplicity of pulsed RF signal sources are used together in advanced electronic warfare (EW) signal generation systems, parasitic error terms in the signal generation circuits may produce timing skews between channels. It is therefore necessary to align the channel to channel timing, so that when pulses are commanded to occur at the same time, they are exactly coincident, for example, to sub-nanosecond accuracy.
An exemplary radio frequency (RF) pulse has characteristics illustrated in FIG. 1. As shown, the pulse consists of a sinusoidal carrier frequency that is gated on and off to form the pulse. Because the on/off gating is not instantaneous, both the leading and trailing edges of the pulse have exponential rise/fall times, though the slew rates are typically different. As indicated in the illustration of FIG. 1, the rise time is typically much slower than the fall time. The characteristic results from the physics of the devices used to switch the pulsed carrier on and off.
The exact time of occurrence of an radio frequency (RF) or microwave pulse is very difficult to measure to sub-nanosecond accuracy. The reason for this is illustrated in FIG. 1, which shows a 1 gigahertz (GHz) RF pulse, with 24 nanosecond (ns) pulse width and a rise time of approximately 7 ns. The leading edge of the pulse is generally defined as the time at which the pulse envelope, illustrated in FIG. 2, passes the fifty percent (50%) point. However, as can be seen from FIG. 1, the pulse envelope is sampled by the carrier frequency, which is an oscillating signal, and the instant the envelope passes through the 50% point almost certainly occurs between carrier cycles.
Schottky detector diodes can be used to detect the envelope of the RF pulse, and the detected video output from the diode is compared to the 50% voltage threshold to determine the pulse time of arrival. This legacy approach works well if the allowable error in timing measurements is greater than 5 to 7 ns. However, for higher accuracy measurements (for example, less than 1 ns), and particularly for measurements across a wide band of frequencies, detector diodes are insufficient.
Detector diodes have time constants of approximately 3 ns, and are frequency dependent. Furthermore, the match from diode to diode is not particularly predictable, with the result that the matching of detected waveforms on two channels of an oscilloscope does not guarantee that the pulse times are necessarily coincident to sub-nanosecond accuracy. Video ringing on the detected pulses is yet another distortion mechanism that limits the accuracy at which pulse timing can be measured.
RF pulses below a few GHz can be directly viewed on high speed sampling oscilloscopes, and the point at which the envelope crosses the 50% threshold can be determined by interpolating the captured data. This approach is only good for relatively low frequencies, however, as the best digital scopes available today typically cannot directly sample frequencies above about 5 GHz. Furthermore, the effects of limited sample rates tend to limit the accuracy that may be obtained from interpolation.
An alignment receiver may be used to achieve RF pulse timing, where the receiver receives signal sources under test RF pulses and aligns the pulses in phase and in amplitude. However, such systems have the limitation that they have little or no real capability to measure pulse timing characteristics. Thus, while an alignment receiver may provide a means to accurately measure and align phase and amplitude of an RF carrier signal, the measurements of pulse timing must be made by other instrumentation.